In recent years, various technologies have been employed in order to flatten any curled recording sheets, which technologies are shown below.
(1) As general technologies, a de-curling device, including a paired-roller set formed of a soft roller and a hard roller, is well known, in which a curled recording sheet is conveyed through the soft roller and the hard roller so that the curled recording sheet is flattened. However, a curled recording sheet shown in FIG. 8a, which is curled perpendicular to the sheet conveyance direction, (hereinafter referred to as an “orthogonal curl”) can be effectively flattened, but a curled recording sheet shown in FIG. 9a, which is curled parallel to the sheet conveyance direction (hereinafter referred to as a “parallel curl”), cannot be flattened, and a curled recording sheet shown in FIG. 10a, which is obliquely curled, cannot be flattened (hereinafter referred to as an “oblique curl”). Further, in order to flatten various recording sheets, which are curled in the opposite direction as shown in FIGS. 8b, 9b, and 10b, a switching operation to flatten said opposite curled sheet is not prepared in said de-curling device.
(2) Still further, Unexamined Japanese Patent Application Publication Number 2002-241,021 discloses a de-curler, in which a spiral roller and a normal roller are aligned in parallel to press to each other, so that a recording sheet sandwiched between them is flattened. In this de-curling device, spiral structures are symmetrically formed from the center to each end of the spiral roller to flatten the curled recording sheet.
However, the effect of the above technologies are adversely limited, depending on curling directions (such as a positive curl, and a negative curl), curling formations (being the orthogonal curl, the parallel curl, and the oblique curl), and curling amounts. That is, though the curl shown in FIG. 8a is effectively flattened, it is very difficult to flatten the various curls shown in FIGS. 8b, 9a, 9b, 10a, and 10b. 
(3) Still further, Unexamined Japanese Patent Application Publication Number 5-341,600 discloses a de-curling device in which a nip section is formed of a belt and a nipping roller, and a curled recording sheet is conveyed through the nip section so that the curled recording sheet is flattened. However, the effect of the above technology is adversely limited, being the same way as the case of item (2).